1. Field of the Invention
The present invention relates to a carbon fiber and a process for producing the same. More particularly, it relates to a carbon fiber made from an acrylic fiber having an excellent compressive strength, and a process for the production of this carbon fiber.
2. Description of the Related Art
With the recent increase in the use of carbon fibers, the requirements for carbon fibers have become very strict. The main requirement has been directed to the tensile characteristics, and therefore, the tensile strength has been greatly increased. Nevertheless, the compressive strength is little improved, and therefore, the problem of suppression of increase of practical characteristics, such as flexural strength, due to the low compressive strength has become serious. In a graphite fiber having an elastic modulus of at least 390 GPa, formed by heat treatment at a high temperature, i.e., having a large crystal size Lc, the compressive strength of a single filament is about 3.5 GPa. This value is as low as about 1/2 of the compressive strength (7 GPa) of a single filament of a carbon fiber having an elastic modulus of 245 GPa. This is a serious problem.
Many proposals have been made for techniques of improving the tensile characteristics, but very few proposals have been made for techniques of improving the compressive strength.
A graphite fiber having a high compressive strength and a high elastic modulus of at least 340 GPa has been proposed, which is formed by specifying the spinning and heat-treating conditions (Japanese Unexamined Patent Publication No. 63-211326).
A chemical oxidization treatment of a carbon fiber with a hot concentrated inorganic acid such as sulfuric acid, nitric acid or phosphoric acid, or an electrochemical oxidation treatment of a carbon fiber in an aqueous solution of an electrolyte containing a nitric acid ion and a subsequent inactivating treatment has been proposed (Japanese Unexamined Patent Publication No. 58-214527 and Japanese Unexamined Patent Publication No. 61-225330) as a technique for reducing the crystallinity of the surface layer. Each of these proposals effectively improves the tensile strength, but does not greatly improve the compressive strength. Further, in the above-mentioned treatments, an excessive amount of oxygen-containing functional groups are formed in the surface layer of the carbon fiber, and since the functional groups are removed by the treatment, an inactivating treatment, which costly, must be carried out.
The technique of accelerating an ionized atom or molecule and implanting the same in the surface of a material, i.e., the ion-implanting method, has been examined as a technique for modifying the structure of the surface layer portion, mainly in the field of semiconductors (Japanese Unexamined Patent Publication No. 58-87818 and Japanese Unexamined Patent Publication No. 58-87894).
It also has been proposed to implant an ionized atom or molecule in a carbon material (Japanese Unexamined Patent Publication No. 62-235280).
In connection with the ion implantation into a carbon fiber, an ion implantation in a vapor-phase grown carbon fiber was reported (TANSO, No. 104, page 2, 1984), but in the case of a carbon fiber having a high anisotropy, such as a vapor-phase grown carbon fiber, is subjected to the ion implantation treatment, a noticeable improvement of the compressive characteristics, as obtained in an acrylic carbon fiber, cannot be obtained.